I need some help setting up my 2x systems (system 1 with 2x 6950's and system 2 with 2x 5830's) with GUIMiner.Should I assign individual usernames and passwords (with each card setup with different names in the mining pool website) to each card or the same username and password to all cards (with only one miner registered on the mining pool site)?

I suggest using a different worker-name for each card, since it's way easier to spot problems that way. If you had them all using the same worker-name, and you were getting 200mh/s lower speeds than expected then it would be really had to spot which card is causing the problems from the pool website.

I suggest using a different worker-name for each card, since it's way easier to spot problems that way. If you had them all using the same worker-name, and you were getting 200mh/s lower speeds than expected then it would be really had to spot which card is causing the problems from the pool website.

I've personally had problems with multiple cards registering hashes under a single worker name - I suggest opening an individual client for each card, and as such registering a separate worker for each card.

As fascistmuffin said, this also comes with being able to spot problems much more quickly than if you shove as much as you can onto a single profile.